1. Field of the Invention
The present invention relates to a synthesizer for use in a transmitter/receiver in a microwave satellite communication system or the like.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows a conventional synthesizer for use in a transmitter/receiver. The conventional synthesizer shown in FIG. 1 comprises a direct digital synthesizer (DDS) 20 and a bandpass filter 24. As shown in FIG. 2 of the accompanying drawings, the DDS 20 comprises an adder 21, a waveform table 22 which generally comprises a read-only memory (ROM), and a digital-to-analog (D/A) converter 23.
When oscillation frequency data DI (of n bits) is supplied to the adder 21, the adder 21 cumulatively adds the supplied oscillation frequency data DI in synchronism with a clock signal CK having a frequency f. Based on the sum from the adder 21, the waveform table 22 produces a sine-wave waveform data which are supplied to the D/A converter 23 which generates a sine-wave output signal. The sine-wave output signal from the D/A converter 23 passes through the bandpass filter 24, which produces an output signal DQ.
The conventional synthesizer shown in FIGS. 1 and 2 offers the following advantages:
(1) Since the accuracy of the frequency of the output signal DQ is governed by the accuracy of the frequency f of the clock signal CK, if the clock signal CK is of low phase noise, then the output signal DQ of the synthesizer 2 is of the same low phase noise level.
(2) The frequency of the output signal DQ can quickly be changed into any frequency without interrupting the output signal DQ.
(3) When the oscillation frequency data DI is varied, the digital signal can directly be frequency-modulated.
However, the conventional synthesizer also suffers the following shortcoming:
The bandpass filter 24 has a fixed passband which is required to be wide enough to pass the output signal from the DDS 20 which varies as the oscillation frequency data DI varies. The bandpass filter 24 of such a wide passband is however unable to eliminate spurious signals sufficiently from the output signal.